Energizing system for discharge tubes



Oct. 25, 1938. E, G, DQRGELO 2,34,439

ENERGIZING SYSTEM FOR DISCHARGE TUBES Filed Dec. 27, 1957 E 6 oilyeZ, O

Patented Oct. 25, 1938 UNITED STATES PATENT OFFICE ENERGIZING SYSTEM FOR DISCHARGE TUBES Application December 27, 1937, Serial No. 181,929

i In Germany December 29, 1936 3 Claims.

The present invention, which forms a continuation-in-part of my copending U. S. patent application Ser. No. 127,756, now Patent #2,130,077, relates to a system for periodically energizing high-pressure metal-vapor discharge tubes, particularly when such tubes are used as lightsources in motion picture apparatus.

My invention is especially useful with discharge tubes of the type described in U. S. Patents #2,094,694 and #2,094,695 to Cornelis Bol et al. Such tubes are particularly suited for projection purposes because they develop an intrinsic brilliance up to and above 200,000 Int. candles per sq. in., and the spectrum of the emitted light meets all the requirements of picture projection. Furthermore, such tubes have the advantage that when they are properly energized, their light-emission -curve has dark periods of such duration that the nlm can be moved during these periods without the movement being visible on the screen, which makes a shutter unnecessary.

In my above-mentioned application I have described an energizing system for such tubes which comprises a circuit connected across an alternating-current supply and having twoparallel branches. One of these branches comprises a preferably variable ohmic resistance and a rectier, the other branch comprises an impedance and the discharge tube, and a condenser is connected in series with both these branches.

I have found, however, that the life of such tubes depends upon the manner in which the current passes through them, and if this current is periodically interrupted, the life of the tube is materially reduced. More particularly, due to` the reduction of the ionization in the discharge space of the tube, the starting voltage of the tube is increased, which may result in premature destruction of the tube. Furthermore, to maintain a given mean intensity of illumination, it is necessary that the tube has a higher luminous intensity during the periods of light emission, i. e. a larger current must be passed through the tube. Because of the above the life of the tube was considerably shortened. Y

The main object of my invention is to overcome the above diiliculties and to increase the life of such tubes. cordance with the invention, I use in circuits of the above type a condenser and an impedance of such values that a current will continuously pass through the tube.

I make the minimum value of the tube current such that the emission of light from the tube periodically becomes so 10W that the movement' For this purpose and in ac- (Cl. 17E-Ml) of the lm can be effected during this period without being perceptible on the screen, and thereby make the use of shutters unnecessary.

It is well known that, in shutterless apparatus, it is unnecessary that the light emission be reduced to zero during the periods of 'movement of the nlm. On the contrary, if the luminous intensity on the projection screen during movement of the nlm falls below a given value which depends on the maximumv value of the luminous intensity, a travel ghost of the picture is not perceptible for physiological reasons. When the projection is carried out with relatively small variations in luminous intensity, for example, in comparatively small systems, a shutter may be provided. In such cases the device according to the invention not only prolongs the life of the discharge tube but also effects a certain saving in current.

In order that my invention may be clearly understood and readily carried into effect, I shall describe the same in more detail with reference to the accompanying drawing, in Which:-

Figure l is a schematic circuit diagram of an arrangement according to the invention,

Fig. 2 is a sectionalized view of the discharge tube of Fig. l,

Fig. 3 is a schematic circuit diagram of an arrangement according to another embodiment of the invention, and

Fig. 4 is a graph showing the currentand voltage-time curves of the arrangement of Figure 1 whenthe condenser and the impedance have the values in accordance with the invention.

As shown in Figure 1, an alternating-current source e, which may be the ordinary lighting or power supply, or a transformer, has one terminal l connected through a condenser C to a point 1, and has its other terminal 2 connected to a point 8. Connected in parallel across points l and 8 is a. branch circuit comprising a rectifier G and a variable resistance R, and a second branch circuit comprising a choke coil L and a discharge tube E. Resistance R may be connected between points 3 and 4, or between points 5 and 6 as shown in Fig. 3. In addition, the choke coil L may be replaced by a resistance.

As the discharge tube E, I prefer to use cooled high-pres`sure metal-vapor discharge tubes such as described in the above-mentioned patents. Preferably the tube should be lliquid-cooled and have a mercury-vapor filling of high-pressure, preferably more than 6 atmospheres, for eX- ample 150 atmospheres, and should comprise one or more incandescible electrodes protruding il l) only slightly from a vaporizable metallic mass which surrounds said electrodes and which contains mercury or amalgam. With such discharge tubes an intrinsic brillianceof 20,000 International candles per sq. cm. and upwards, e. g. from 80,000 to 100,000 and more, can readily be obtained, while the spectral composition of the light fully meets the requirements to be complied with for faultless projection. Such a tube is illustrated in Figure 2 in which the reference numeral 3| indicates a transparent envelope, e. g. of quartz, in which are disposed two mercury electrodes 32. The tube is surrounded by a cooling jacket 33 of transparent material through which cooling water is circulated as indicated by the arrows.

The circuit arrangement operates as follows. When terminal 2 is positive, condenser C becomes charged Aby a potential which is governed by resistance R, since rectifier G permits the current to ilow in this direction. When the potential oi terminals I and 2 are reversed, there is consequently set up across tube E a total voltage which is the sum of the voltage of condenser C and the voltage of supply e. It follows therefore that the circuit arrangement illustrated in Figs. 1 and 3 makes it possible to start and to maintain in operation discharge tubes which have a starting-voltage higher than the highest voltage of the alternating-current source.

The operation of the circuit arrangements shown in Figs. 1 and 3 is illustrated by the graph of Fig. 4 in which curve 9 represents the voltage curve of the alternating-current source e. When the supply voltage reaches a value corresponding to point I0 on curve S-which point corresponds to the breakdown voltage of rectifier G-the latter becomes operative and condenser C cornmences to be charged. Whether or not condenser C will become charged to the highest value that can be reached, i. e. to the maximum A. C. voltage indicated by point I2, depends upon the capacity of condenser C and on the value of resistance R that limits the rate of flow of the charging current. In the circuit arrangement illustrated the value of resistance R is such that the condenser voltage, designated by dotted curve II, approaches very closely to the maximum voltage of the alternating-current source. The condenser retains its charge, because rectifier G does not allow passage of current in the reverse direction. After the voltage of the A. C. source has reached the maximum value at l2, it decreases so that there is set up between the points 1 and 8 of Figs. 1 and 3, a potential difference which is indicated diagrammatically in Fig. 4 by the cross-hatched part.

Connected in series across points l and 8 are the discharge tube E and the choke L. When, after the device has been placed in operation, the voltage across points 'I and 8 reaches for the first time, at point I4, a value equal to the breakdown voltage of the discharge tube, indicated by line I3 in Fig. 4, the tube 'lights up. The iiow of current through the tube is indicated diagrammatically by the curve I6 which starts at a point I5. The condenser then discharges through the tube E as indicated by the portion Il of curve II.

At a point I8, the potential difference between the points 'I and 8 becomes equal to zero. However, the current passing through the discharge tube is not equal to zero at this moment, because the current in choke II lags in well-known manner with respectI to the voltage applied across -described process is then repeated periodically with a frequency equal to that of the alternatingcurrent supply.

If, in accordance with the invention, condenser I C is given a sufliciently high capacity, it will periodically discharge only a comparatively small amount which means that the voltage across -the same drops only a small amount (see curve part Il), and when choke L is given the proper inductance, it is possible to maintain the flow of current through tube E until the voltage between the points I and 8 again increases and a renewed increase in current is set up from the point 23 onwards. Thus tube E will not become extinguished but will be supplied with an uninterrupted and periodically varying current. The higher the values of the inductance of choke L and the capacity of condenser C, the less will be the periodical decrease of the current through the tube.

In the above system the condenser C, the rectiiler G and the impedance L act as a rectifying circuit, in which C and L together form the smoothing filter. The greater the value of the condenser C and the impedance L, the smaller will be the alternating current ripple on the direct current.

When the maximum value oi the A. C. is greater than the D. C. the rectiiied current will beperiodically interrupted; but when it is smaller, the rectified current will be uninterrupted.

In accordance with the invention the condenser C and the impedance L are so proportioned that the maximum value of the alternating component (curve I 6) of the rectified current will be smaller than the value of the direct component (curve 24) of said rectified current, i. e., so that the current through the tube E will not be interrupted.

As a specific example, I have used the following values: Supply voltage e=500 volts; supply frequency f=50 cycles per sec.; operating voltage of discharge tube e=500 volts; a mean tube current 1 or 2 amps.; capacity of condenser C=30 nf; inductance of choke L=from 2 to 3 henries; a resistance R=about from 10 to 100 ohms. With such values, it was found that the life of the tube was about 10 times greater than when supplied with periodically interrupted current.

While I have described my invention in connection with specific examples-and applications, I do not wish to be limited thereto but desire the appended claims to be construed as broadly as permissible in view of the prior art.

What I claim is:

l. An energizing system for a high-pressure metal-vapor discharge tube having a contracted discharge path, comprising a source of alternating current, and means between said source and said tube for passing through said tube an uninterrupted and periodically varying operating current. said means comprising a circuit conacross said supply and having two parallel` branches, a condenser in 'said circuit and in l series with both of sai. branches, a rectiiier in one of said branches, an impedance in said circuit between said rectier and one end of said source, and a second impedance, said second impedance and discharge tube being connected y in series in said second branch, said condenser and second impedance being proportioned to cause an uninterrupted and periodically varying current to pass through said tube.

3. An energizing system for a high-pressure metal-vapor discharge tube having a contracted discharge path, comprising a source of alternating current, and means associated with said source to rectify said alternating current and to supply to said tube an uninterrupted and periodically varying operating current, said means comprising a vcircuit connected across said supply and having two parallel branches, a condenser in said circuit and in series with both o said branches, a rectier in one of said branches, a variable resistance in said circuit between said rectifier and one end of said source, and an impedance, said impedance and discharge tube being connected in series in said second branch, said condenser and said impedance being proportioned to cause the maximum value of the alternating component of the reotied current to be smaller than the value of direct component of said rectied current.

EDUARD GERARDUS DORGELO. 

